Optimum CG Range

On 15 Dec, 15:42, Andy wrote:
On Dec 15, 6:52*am, wrote:

Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

It's not quite that simple though is it?

For the ASW-28, and probably other modern gliders, the "manufacturer's
approved CG range" is dependent on the glider mass. * Again for the
28, a cg position of 75-80 of approved range at min weight (315-321 mm
aft of root leading edge) will be behind the approved aft CG limit at
max gross wt (306mm).

I used to think that the change in aft cg limit with increasing mass
was to protect for the case where the tail tank fails to dump. *If
that is true then ASW 28 built without the optional tail tank would
not have the variable aft limit. *Do they?

Comments or other explanations of the variable aft limit?

Hank - Where is your 28 CG at max gross or at the max weight you fly
at if lower?

Andy (GY)

Are you sure you are reading the manual right? I own a 27 and the aft
limit remains the same. Waibel argued that it was by design that the
CofG moves forward when adding ballast and that this automatically
made for more efficient high speed flight when flying with high wing
loading. He even stated that the fin ballast tank was unnecessary. It
is possible that the practical aft limit for CofG position when
ballasted is well forward of the position and aft limit when empty for
this reason.

Also, if you have a tail tank then it might be wise to ensure that
filling the tail tank only keeps the C of G within limits if there is
any possibility of it not emptying when you dump ballast.

I don't have a 28 manual to look at, have you got one in electronic
form?

On the subject generally. I would recommend flying the glider (within
manufacturers limits) with a CofG that you find best suits your style
and ability. This can be achieved by experimentation. As I understand
it, moving the CofG back improves efficiency at low speeds and in
thermals by reducing the necessity for the tailplane to produce
downwards lift (and drag) in those phases of flight. In extremis it
allows sufficient elevator authority to fly near the stall in this
configuration. The downside is the reduced stability in pitch which
could lead to less efficient handling and pilot induced losses.

Jim

On Tue, 15 Dec 2009 09:56:22 -0800 (PST), jimboffin
wrote:


Are you sure you are reading the manual right? I own a 27 and the aft
limit remains the same. Waibel argued that it was by design that the
CofG moves forward when adding ballast and that this automatically
made for more efficient high speed flight when flying with high wing
loading. He even stated that the fin ballast tank was unnecessary. It
is possible that the practical aft limit for CofG position when
ballasted is well forward of the position and aft limit when empty for
this reason.

Unfortunately he neglected the fact that especially the 27 with its
tiny horizontal tail is usually flown with very high wing loading,
hence especially the 27B really benefits from its tail ballast tank.


On the subject generally. I would recommend flying the glider (within
manufacturers limits) with a CofG that you find best suits your style
and ability. This can be achieved by experimentation.

100% agree.

As I understand
it, moving the CofG back improves efficiency at low speeds and in
thermals by reducing the necessity for the tailplane to produce
downwards lift (and drag) in those phases of flight.

In the 27 you can really notice this - flying a 27 with a forward CG
badly affects its climb performance, yet increases its perfomance the
more the fster you fly.



Bye
Andreas

On Dec 15, 10:56*am, jimboffin wrote:
Are you sure you are reading the manual right?

Don't need to read it. The allowable mass/cg envelope is depicted
graphically on page 5.10 of the flight handbook. I only have the
German version on hand, the English translation is in the glider. The
graph shows an aft limit of 345mm from 300Kg to 380kg, then reducing
linearly to 322 mm at about 460 kg then linearly to about 306 mm at
525 kg max gross.

The envelope also shows that the forward limit moves forward above
380kg but the shift is much smaller than for the aft limit. It also
clearly shows that the aft end of the recommended cg range of
300-310mm falls outside the aft cg limit at max gross.

Hank - what is on page 5.10 of your 28 flight handbook? If it is a
mass/cg envelope what is the allowable CG at 525kg? I'm surprised
that 2 28's both with tail tanks would have different limits. Mine is
serial 28048.


Andy (GY)

On Dec 15, 11:14*pm, Andy wrote:
On Dec 15, 10:56*am, jimboffin wrote:

Are you sure you are reading the manual right?

Don't need to read it. *The allowable mass/cg envelope is depicted
graphically on page 5.10 of the flight handbook. *I only have the
German version on hand, the English translation is in the glider. The
graph shows an aft limit of 345mm from 300Kg to 380kg, then reducing
linearly to 322 mm at about 460 kg then linearly to about 306 mm at
525 kg max gross.

The envelope also shows that the forward limit moves forward above
380kg but the shift is much smaller than for the aft limit. *It also
clearly shows that the aft end of the recommended cg range of
300-310mm falls outside the aft cg limit at max gross.

Hank - what is on page 5.10 of your 28 flight handbook? *If it is a
mass/cg envelope what is the allowable CG at 525kg? *I'm surprised
that 2 28's both with tail tanks would have different limits. *Mine is
serial 28048.

Andy (GY)

OK oops- I have to admit not paying large amount of attention to
variable CG since I don't remember the last time I flew above about
480kg. Easterner ya know. I suspect this limitation relates to Waibel
trying to ensure not going out the back if mains dump and tail
doesn't. Gotta admit it doesn't interest me enough to do the math.
Review does show that empirically through flight experience, I've
pretty much confirmed that the factory suggested range works well.
UH

On Tue, 15 Dec 2009 07:42:51 -0800, Andy wrote:

For the ASW-28, and probably other modern gliders, the "manufacturer's
approved CG range" is dependent on the glider mass.

That's not just for modern gliders and not necessarily connected with the
amount of ballast on board or the pilot's weight.

The Libelle 201B handbook contains a W&B diagram on page 6 (of both
German and English sections). This shows that the GC limits for the
aircraft move forward as its empty weight increases.

I don't understand why this would be the case - just that its documented
in the handbook.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Andy wrote:
On Dec 15, 6:52 am, wrote:


Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH



It's not quite that simple though is it?

For the ASW-28, and probably other modern gliders, the "manufacturer's
approved CG range" is dependent on the glider mass. Again for the
28, a cg position of 75-80 of approved range at min weight (315-321 mm
aft of root leading edge) will be behind the approved aft CG limit at
max gross wt (306mm).

I used to think that the change in aft cg limit with increasing mass
was to protect for the case where the tail tank fails to dump. If
that is true then ASW 28 built without the optional tail tank would
not have the variable aft limit. Do they?

Comments or other explanations of the variable aft limit?

Hank - Where is your 28 CG at max gross or at the max weight you fly
at if lower?
The flight manual for my ASH 26 E also shows a reduction in the aft cg
limit above a certain mass, and a far greater change in the forward cg
limit over the entire mass range. It also shows the "Favorable CG range
for optimum straight flight performance" that is quite broad, about 80%
of the permissible range at full gross, and about 40% at the lowest
weight (100 pound pilot!). There is no chart for "optimum"
maneuverability in thermals, nor is there a tail tank.

It's a flapped ship, so I suspect it has a larger range than comparable
unflapped gliders, regardless of the criteria chosen.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

On Dec 15, 10:42*am, Andy wrote:
On Dec 15, 6:52*am, wrote:

Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

It's not quite that simple though is it?

For the ASW-28, and probably other modern gliders, the "manufacturer's
approved CG range" is dependent on the glider mass. * Again for the
28, a cg position of 75-80 of approved range at min weight (315-321 mm
aft of root leading edge) will be behind the approved aft CG limit at
max gross wt (306mm).

I used to think that the change in aft cg limit with increasing mass
was to protect for the case where the tail tank fails to dump. *If
that is true then ASW 28 built without the optional tail tank would
not have the variable aft limit. *Do they?

Comments or other explanations of the variable aft limit?

Hank - Where is your 28 CG at max gross or at the max weight you fly
at if lower?

Andy (GY)

My glider is set up for about 75% dry- about 315mm. Water goes in tail
tank to maintain this position when water goes in wings. My manual
doesn't have variable limit but does suggest a dry position of 300-310
mm.
My glider is also set up to retain relationship of tail to main water
during dumping.
Only exception is ridge flying when I use less tail weight to unload
ship in gusts.
FWIW
UH

wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.
Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

How steep is the turn for which you still desire the ability to stall?
And how close to the ground or hillside do you expect to need this ability?

It's not clear to me this would be a useful ability for a pilot that is
trying to thermal away from a hillside, mountain, or during a low save.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

On Dec 15, 1:35*pm, Eric Greenwell wrote:
wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.
Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

How steep is the turn for which you still desire the ability to stall?
And how close to the ground or hillside do you expect to need this ability?

It's not clear to me this would be a useful ability for a pilot that is
trying to thermal away from a hillside, mountain, or during a low save.

If you never fly faster than 25 second turns -- and a lot of us don't
-- then it really isn't an issue.

When I'm on my game, I can thermal with 14 second turns, full of
water. 12 seconds, dry. Good trick to have in the bag when presented
with an usually strong, tight core (are you reading, #44? -- good
fun!). And yeah, the cg has to be in the right place to do that.

"Ability to stall" is only an indicator that the ship is set up
correctly.

-Evan Ludeman / T8

On Dec 15, 1:35*pm, Eric Greenwell wrote:
wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.
Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

How steep is the turn for which you still desire the ability to stall?
And how close to the ground or hillside do you expect to need this ability?

It's not clear to me this would be a useful ability for a pilot that is
trying to thermal away from a hillside, mountain, or during a low save.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

I want to be able to fly at 45 deg bank without running out of
elevator. I rarely thermal steeper than that because sink rate rises
and I'm mostly interested in the smallest circle.
Note that I am NOT advocating far aft CG position- actually quite the
opposite.
FWIW
UH